"Flextensional" transducers such as those illustrated in U.S. Pat. Nos. 3,274,537 and 3,277,433 issued to W. J. Toulis on Sept. 20, 1966 and Oct. 4, 1966, respectively, are known in the art and in general, comprise a shell or housing which is excited by a piezoelectric or ceramic transducer stack driven in a length-expander mode which is placed in compression between opposing interior walls of the shell. The elongation and contraction of the transducer stack imparts a motion to the shell, which in general, radiates or couples energy into a surrounding fluid medium such as water.
In such cases, the medium into which energy is coupled is in contact with the exterior surface of the shell and it is the movement of the exterior surface of the shell which couples energy into the surrounding fluid medium.
While such systems are, in general, eminently satisfactory for the purposes intended, when these devices are utilized at significant ocean depths, it is oftentimes desirable to utilize pressure compensation to eliminate the effects of the increased hydrostatic pressure on the transducer shell, and thus, avoid tensile stress in the piezoelectric stack. It will be appreciated that hydrostatic pressure imparted to the stack can, in some cases, result in impaired transducer characteristics.
In order to eliminate the need for pressure compensating systems in moderate depth applications, and to convert large amplitude flextensional motion to a piston-like motion so as to enable the utilization of flat or cylindrical radiating surfaces, it is possible to utilize the interior surface of the flextensional transducer shell and couple this motion through an internally contained fluid to a piston-like or diaphragm-like surface which has an exposed face in contact with the fluid medium into which energy is to be projected. In reverse, acoustic energy arriving at the transducer through the fluid medium is coupled through the internal fluid to the shell of the flextensional transducer and creates an output voltage from the ceramic or piezoelectric transducer stack proportional to the magnitude of the signal.
In one embodiment, the subject transducer utilizes a flextensional transducer sealed at one end. The opposite end of the transducer is coupled to a chamber sealed at one end with a flat or cylindrical radiating surface, and the entire unit is filled with fluid such as oil.
In the transmit mode, as the flextensional transducer shell flexes, the internal fluid transmits the motion of the interior surfaces of the flextensional transducer into motion of the flat or cylindrical radiating surface, which, in turn, projects the acoustic energy into the surrounding fluid medium.
This type of transducer can be utilized for a frequency band about the resonance of the flextensional transducer, or with increased bandwidth about a region below the flextensional resonance. In an alternative embodiment, increased deflection of the exterior radiating surface can be achieved by adding flextensional modules as the prime drive mechanism, with the flextensional modules being driven in parallel. In this embodiment the flextensional transducer opens up into a large chamber which supports an increased radiating surface.
The subject transducer eliminates the need for pressure compensation and results in improved aging under storage conditions, in that there is no requirement for prestressing the ceramic stack. As a result of hydrostatic compression of the ceramic stack there is an extended depth capability because the internal fluid is at ambient pressure. Moreover, since ceramic prestress increases with depth, in contrast to decreasing prestress for air backed designs, the shelf life is not effected by long term application of relatively high stress levels. Shell creep is also a non-issue in this design.
In summary, the subject system involves the utilization of an "inverse" flextensional transducer in the sense that it is the interior surface of the flextensional transducer shell the motions of which produce the radiation of acoustic energy.
The advantages of this configuration are that it eliminates the need for pressure compensation or complaint tubebacking; results in improved againg characteristics for ceramic stack and shell creep; results in extended depth capabilities; and provides that stress increase in ceramic with depth rather than decrease as in air-backed designs. The design also results in radiation loading determined by exterior surfaces and potential improved bandwidth and volume velocity within a constrained radiating surface. There is, of course, greater reliability through the utilization of a self-contained unit and it should be noted that the ceramic is in direct contact with a cooling agent. Finally, new shell materials are practical for lower frequencies without introducing heat transfer problems.
It is, therefore, an object of this invention to provide an improved electro-mechanical transducer.
It is another object of this invention to provide an inverse drive flextensional transducer which incorporates all of the above advantages.
It is another object of this invention to provide a method for utilizing a flextensional transducer so that the internal surface of its shell is utilized as the prime drive mechanism.
These and other objects will be better understood when taken in connection with the detailed description and the drawings appended hereto wherein: